Modern automatic transmissions generally utilize a torque converter or fluid coupling to provide a slipping drive connection between an engine and a multi-speed gear arrangement. The main purpose of the torque converter is to provide increased torque multiplication during starting of a vehicle. The torque converter is also useful in restricting the amount of engine torsional vibrations that can be transmitted to the gearing arrangement and therefore felt by the operation.
However, at higher vehicle speeds and engine speeds, the torsional vibrations are less harsh and it is then desirable to eliminate the torque converter or fluid drive. This has generally been accomplished by the use of a friction type clutch which has a spring damper included therewith to provide for reduced vibration transmission.
It has also been known to utilize a viscous clutch in serial arrangement with the friction clutch to provide continued damping between the engine output and the gearing input. The viscous portion of the clutch is generally a sealed arrangement, such that the transfer of fluid into and out of the viscous clutch is not permitted. Accordingly, the viscous fluid can have a significantly different pressure than the pressure of the fluid available in the torque converter, which is generally a hydraulic type fluid.
It is also known that it is not wise to intermix these two fluids as this reduces the torque transmitting capacity of the viscous fluid in the viscous portion of the clutch.